Coupling of surface-upgrading films to manufactured articles made of thermoplastic materials is a known technique in the injection molding technology. The technique is utilized to confer finished articles an external surface exhibiting improved mechanical, aesthetic or decorative characteristics.
Said technique can be carried out by two methods: the first method involves positioning in the mold a polymeric film supporting a layer of surface-upgrading material, which is transferred from the film onto the external surface of the manufactured articles during the production process; the second method is based on using films which consist themselves of the surface-upgrading material and which remain adhered to the molded object as a result of the melting of their surface layer in contact with the manufactured article.
In both of the above described methods, high temperature/pressure conditions typical of injection molding are required, in addition to chemical compatibility between the materials, in order to achieve a permanent adhesion of the surface-upgrading material to the manufactured article.
The film can be positioned in the mold by different techniques, e.g. unwinding of reels placed in proximity of the mold, or performing a direct deposition in the mold using automated devices similar to those adopted for labelling of bottles. In both cases, the film is held in the mold by appropriate techniques, e.g. by applying vacuum or by effect of electrostatic charges.
With this technique manufactured articles are produced such as wheel covers for motor vehicles, panels made of artificial wood or artificial marble, articles for cosmetics, etc.
However, the utilization of the upgrading films presently on the market can less feasibly be proposed for the conventional blow molding and thermoforming technologies, since these technologies are characterized by remarkably lower temperatures and pressures than in injection molding.
An alternative method to overcome this limitation could have been that of improving chemical compatibility between upgrading materials and polymers used to mold the article, e.g. chemical etching, flame treatment, exposure to a corona discharge, application of primers, etc.
Experience has however demonstrated that in the case of blow molding or thermoforming these compatibilization techniques, in addition to increasing the production costs, have not given good practical results in terms of long term adhesion.